Scratch build PRUSA MK3 slightly larger

Hello I am new to the form and 3d printing. I am planing on building a PRUSA MK3 clone. Nothing is written in stone yet. Current idea:
Basically a cone but 300x x 300y x 400z mm using 12mm smooth rods.
Does anyone see any mechanical issues doing this? particularly with the bed.
What is the weight of the bed I should stay under with 3 bearings?
For cost is there any sizes I should or should not do because part pricing or locating. (saw the mega PRUSA but parts pricing was getting pretty high)
Plan on using a J6 head any recommendations? cheap reliable? (would like to be able to upgrade to a dual filament feed system)
bed heaters reliable sources?
What is the maximum weight requirements for z using 8mm Acme screw and the 17 nema motor.
Accuracy and quality is important so I will reduce size to keep accuracy and quality.
I have done some research but there is a lot of data and some is pretty dated. so any links for good/new resources would be helpful.

I won't comment on the mechanics. But you will need at least 270W of bed heater power and preferably 360W to heat a 300 X 300mm bed in a reasonable time. This is not practical using 12V power. One option is 24V power and electronics capable of running at 24V and switching 15A of bed current. So not budget electronics like RAMPS. Another is an AC mains voltage bed heater controlled by a DC-AC SSR, but I don't advise that if the bed will be moving in the Y axis for safety reasons.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks i shoukd be good with a 20-30 amp 24v power supply. Due to power constraints I probably willnot go over 300x300. Thanks did not think of that.

The price range for 10" beds and heaters is much lower than for bigger stuff.
With J6 head you probably mean the j-head E3D V6 clones, which are good enough quality. But you have to live with the bad Karma of buying cloned stuff

Where do you get the printed parts for the non-standard 12mm rods and such?

I do not plan on getting a cloned head. Is the promethious head any good? The printed parts i still have to model up. I may have a coworker that can print them or find someone that can. Any idea on what that will cost to have them printed?
Do you think its worth the effort to enlarge the work area?

Edited 1 time(s). Last edit at 03/02/2018 02:51PM by Schwartzer.

So it looks like i can use the mega PRUSA models for 9 of the pieces i just need to fix 1 model (y axis smooth rod holder)
I also found 24v mega controller and a fix for the ramps to make it 24v . To bad i can only find 1 place that sells the 1.4.2 ramps board. 75 eruo! To much to make it worth while.
I think i need to order some of the hardware to lock in the size.
Smoot rods:
X 450mm
Y 550mm
Z 500mm
X and z may change depending on pricedeals.

Putting it the model together I noticed the mega is using plane thread rod. So all models will need to be updated.